1. Field of the Invention
The present invention relates to a thermoelectric conversion module for use in an apparatus utilizing a thermoelectric effect such as electronic cooling apparatus and electric power generating apparatus, and more particularly to a thermoelectric conversion module having N type semiconductor elements and P type semiconductor elements, which are connected in series with each other by means of metal electrodes and are thermally and electrically isolated from adjacent semiconductor elements by a module core consisting of an electrically insulating material. The present invention also relates to a method of manufacturing such a thermoelectric conversion module.
2. Related Art Statement
When a pair of thermoelectric conversion elements comprising P type semiconductor element and N type semiconductor element, respectively are electrically connected in series and a connected end portion is subjected to a higher temperature and the other end portion is subjected to a lower temperature, there is generated a thermoelectric voltage across these end portions in accordance with a temperature difference. This phenomenon is called the Seebeck effect. In the above thermoelectric conversion element pair, when a current flows from one semiconductor element to the other semiconductor element, one end portion absorbs heat and the other end portion generates heat. This phenomenon is called the Peltier effect. Furthermore, when one of the p type semiconductor element and N type semiconductor element is subjected to a higher temperature and the other is subjected to a lower temperature and an electric current flows along a temperature gradient, heat is absorbed or generated in accordance with direction of the current. This phenomenon is called the Thomson effect.
The thermoelectric conversion module utilizing the above mentioned effects does not includes a movable member which might produce undesired vibration, noise and abrasion, and thus may be advantageously used as a direct energy conversion apparatus having simple construction and long life time and being easily managed. Such a thermoelectric conversion module comprises at least one pair of P type semiconductor element and N type semiconductor element, and usually the element pairs are connected electrically in series and thermally in parallel. The thermoelectric conversion module has been used as an electric power generating apparatus using the above mentioned Seebeck effect, in which a voltage is generated in accordance with a temperature difference, and as a thermoelectric cooling apparatus using the Peltier effect, in which one end portion is cooled by producing a temperature difference in accordance with a voltage applied across both end portions.
FIG. 1 is a schematic view showing a known thermoelectric conversion module. P type semiconductor elements 21 and N type semiconductor elements 22 are arranged in accordance with a given pattern. One end portion of adjacent thermoelectric elements is connected by means of electrodes 26 formed on an insulating substrate 25 and the other end portion of adjacent thermoelectric elements is connected by means of electrodes 27 also formed on an insulating substrate such that successive thermoelectric elements are connected in series. No heat insulating members are provided between adjacent thermoelectric elements and the thermoelectric module has a skeleton structure. It should be noted that in FIG. 1 the insulating substrate for the electrodes 27 is not shown for the sake of simplicity.
In general, the thermoelectric module includes several tens of semiconductor element pairs. As explained above, since the known thermoelectric module has the skeleton structure, it is rather difficult to arrange accurately a number of semiconductor elements in accordance with a given pattern and to connect these semiconductor elements in series and are kept in position until the electrodes are secured to the elements. Further, the known thermoelectric conversion module has a drawback that it could not have a large mechanical strength.
In order to mitigate the above drawbacks of the known thermoelectric conversion module shown in FIG. 1, several proposals have been done for the construction and manufacturing method of the thermoelectric conversion module.
(1) In Japanese Patent Laid-open Publication Kokai Hei (JP-A) 5-283753, there is proposed a structure in which p type and N type semiconductor elements are arranged within holes formed in a heat resisting insulator, and in JP-A 7-162039, there is shown a structure in which semiconductor elements are inserted into holes formed in a mold substrate made of an electrically insulating material.
In such structures, the P type and N type semiconductor elements can be inserted into the through holes formed in the insulating body without using any tool, and therefore a workability is improved, adjacent elements can be effectively isolated from each other, and a mechanical strength of the thermoelectric module can be increased.
(2) There has been also proposed another known structure, in which spaces between adjacent semiconductor elements are filled with an electrically insulating material. For instance, in JP-A 8-18109 and U.S. Pat. No. 4,459,428, there is disclosed a structure in which P type and N type semiconductor elements pairs are embedded in an insulating material. In JP-A 61-263176, there is proposed a structure in which a plurality of P type and N type semiconductor layers are stacked, spaces are formed in portions other than PN junctions, and the spaces are filled with an electrically insulating glass material. In thermoelectric conversion modules having such structures, since spaces between adjacent semiconductor elements are filled, with an insulating material, the mechanical strength of the module is improved and the semiconductor elements can have acid proof and anticorrosion property.
However, in the known thermoelectric conversion module described in the above mentioned item (1), it is difficult to manufacture a thermoelectric conversion module in which a height defined as a distance between a higher temperature end surface and a lower temperature end surface is uniform over a whole surface of the module. In a thermoelectric conversion system in which a number of thermoelectric conversion modules are integrally arranged, it is important to make uniform thermal contact between a heat source and the modules. Therefore, a high accuracy in the height of the thermoelectric conversion modules is important. Particularly, in an electric power generating system formed by thermoelectric conversion modules in order to generate electric power by utilizing an exhausted heat from a combustion system of an automobile, it is necessary to arrange more than several tens thermoelectric conversion modules. In such a system, if the thermoelectric conversion modules have different heights, the temperature difference between end portions of the respective thermoelectric conversion modules is also varied in accordance with a thermal contact condition and output powers generated by respective thermoelectric conversion modules are varied, Then, thermoelectric conversion modules whose output power is low might consume output power of thermoelectric conversion modules generating higher output power. This results in a large decrease in the output power of the total thermoelectric conversion system.
Furthermore, in the known thermoelectric conversion modules described in the above item (1), there are formed air spaces between the semiconductor elements and inner walls of the holes into which elements are inserted, heat might dissipate from side walls of the semiconductor elements into the air spaces and heat transfer might occur within the air spaces. Therefore, a heat current passing through the elements is reduced, and as a result of which the output power is decreased. When the thermoelectric conversion module is applied to the electric power generating system utilizing heat exhausted from an internal combustion engine of an automobile, a higher temperature side is subjected to a very high temperature up to 800.degree. C. and there is produced a large temperature difference between the higher temperature side and the lower temperature side. Therefore, it is particularly important to suppress an undesired reduction in the thermal energy passing through the semiconductor elements as far as possible. In the known construction proposed in the above item (1), in order to reduce an air space between a semiconductor element and an inner wall of the hole, these parts have to be manufactured with an extremely high precision, and therefore an operation of inserting the semiconductor element into the hole requires a high precision and becomes very cumbersome. In this manner, a manufacturing cost would be increased materially.
In the known thermoelectric conversion module proposed in the above item (2), semiconductor elements have to be maintained in position until spaces between adjacent elements are filled with an insulating material. In the thermoelectric conversion module disclosed in JP-A 61-263176, spaces between the semiconductor layers are filled with a glass material having a low melting point such as vitreous enamel. When the higher temperature side of the thermoelectric conversion module is heated to a temperature near the melting point or softening point of the low melting point glass, the glass filled in the spaces between the semiconductor layers is deformed due to a thermal expansion. When the temperature is increased up to the softening point, the glass is softened and deformed to a large extent Particularly, in the electric power generating system producing a large electric current, a current leakage due to the deformation and alternation of the insulating member might cause undesired overheat and a possibility of fire is increased.
In the above mentioned JP-A 8-18109, P type and N type semiconductor films are formed on glass substrates, respectively, and the semiconductor films are selectively removed by a dicing machine to form pillar-like semiconductor elements. Then, the glass substrates are stacked with each other such that the P type semiconductor elements and N type semiconductor elements are arranged alternately. Finally, spaces formed between the glass substrates are filled with an electrically insulating material. In such a method, an amount of semiconductor materials removed from the original semiconductor films becomes larger than that of the pillar-like semiconductor elements, and thus a manufacturing cost is increased.
In the above mentioned U.S. Pat. No. 4,459,428, after fixing the semiconductor elements by soldering electrodes to both ends of the semiconductor elements, spaces are filled with an insulating material. In this method, at first, electrodes are connected to one ends of the semiconductor elements held by a supporting member, then after removing the supporting member, electrodes are connected to the other ends of the semiconductor elements, and finally spaces are filled with the insulating material. In this case, since the electrodes have to be connected to the both ends of the semiconductor elements by different steps, it is necessary to prepare solders having different melting points. In general, a temperature at which electrodes are connected by soldering has to be set to a value within a range from a temperature of a higher temperature side of the thermoelectric conversion module under practical use and a sintering temperature of the semiconductor elements. When the higher temperature side of the thermoelectric conversion module is subjected to a high temperature, said temperature range becomes very narrow and it is practically difficult to prepare two kinds of solders or brazing materials having different melting points.